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When you see a picture with details like this (from my Mac’s background picture)…
…then you know that a small aperture was used for this photo.

The only way to get the sharp star shape you see here, you see, is to use a small lens opening. Meaning a small aperture (“aperture” means “opening”). Meaning a high “f-number”. In this case, I used an aperture of f/22. The reflection is from my flash, which was aimed straight at the car.

I have other clues. Other detail in the picture includes:
That is at least proof that the lens was not wide open. If it had been, the polygon at the top would have been not a polygon, but a circle.

Other notable facts: the lines (there’s your stripes) all converge where the sun is. And finally, the lens is probably an expensive one: the polygon has seven sides. Most have five or six sides. The more sides, the more the lens approaches the ideal, a circle. That ideal gives you great bokeh.

Bokeh

THE TERM BOKEH, by the way, when used correctly, is used to describe the quality of the fuzzy background. “I want bokeh” is not a correct term: when people say this, they usually just mean “I want a blurry background”.

Correct usage: A lens that has great, beautiful bokeh is a lens whose blurry background is wonderfully smooth and evenly creamy. A cheap lens, on the other hand, has bokeh (especially “fully open” bokeh) that is more like clotted cream: much less smooth, more uneven. I can tell a cheap lens from an expensive one immediately, and I bet you can, too, when you see them side by side.

And that concludes today’s lesson. For more, attend one of my many upcoming workshops: scroll down to read more.

If I want a sharp foreground subject with a blurred background – you have heard me say it many times, there are several ways.

The reason this subject is always confusing is that it is very complicated. “Sharp focus” and “depth of field” are subjects for mathematicians (check the Wikipedia entry, if you wish). Hyperfocal distance, lens geometry, approximations, cropping, aperture, magnification, f-numbers, image format size, sensor size – all these have an effect. The main factors that affect DOF are:

Sensor size

Proximity to subject

Zoom

Aperture number

The ratio of subject distance to focal length

Cropping

Several of these factors are complicated and need not be taken into account all the time – but several can help you in practice. Chief among them: it is not just aperture that affects depth of field – it is also the distance to the subject. As Wikipedia puts it:

For a given format size, at moderate subject distances, DOF is approximately determined by the subject magnification and the lens f-number.

In practice, this means that to get less depth of field (i.e. a blurrier background), you need to either:

Select a lower f-number, or…

You need to magnify more. And you can magnify more by zooming in, or by getting closer.

So to get an image like this, with the person behind the object blurred out, you do not necessarily need a fast lens or a full-frame camera:

In fact that was taken with a Canon 7D with a 35mm lens (equivalent to a 50mm lens) set to f/5.6. This is an aperture that every lens can achieve. But I was close to the object!

That said, of course a lower f-number in the same situation gives you more blur. Here’s f/2.8, which good zooms can achieve:

And here’s f/1.4, for which you need a prime lens:

So the lesson, I suppose, is that if you want blurred backgrounds but you cannot right now afford that full frame camera and the low f-number lenses that you should really invest in, at least get close.

A post for beginners, today. About the “F”-number and why it is important. Very important.

The most important number is the minimum f-number a lens can be set to. In other words, the maximum aperture, or lens opening that this lens can go to.

F-numbers? Yes, you know. These numbers: f/16, f/11, f/8, f/5.6, f/4, f/2.8, f/2.0, f/1.4, etc. And the lower the F-number, the larger the opening. (The diameter of the opening, by the way, is the lens’s focal length, f, divided by this number. So a 100mm lens set to f/4 would have an aperture diameter of 100/4 = 25 mm).

So how low can we go?

On your consumer lens, that minimum f-number is 3.5 when you zoom out, or 5.6 when you zoom in. So on your lens it says “1:3.5-5.6”. Look at the top or at the very front of the lens.

On my photojournalist zoom lenses it says “1:2.8”, meaning that the lens can open to f/2.8 whether I zoom in or out.

On my fixed, or prime, lenses, this number says “1:1.4”: i.e. I can open to f/1.4.

This is important for two reasons.

First: lower F-numbers means more selective depth of field, i.e. the ability to blur the background. See these pictures, of a student the other day:

f5.6- 1/60th second

f2.8 - 1/250th second

f1.4 - 1/800th second

You can see clearly that if you want those blurry backgrounds, you need the low f-numbers. No substitute will do. That’s why we pay for “fast lenses” (this just means “lenses with a low “F”-number”).

Second, the larger the opening (the smaller the f-number), the more light comes in. And hence, the faster the shutter speed can be. Look again at the above images. The “consumer lens” needs 1/60th second – barely fast enough for a sharp image. The “pro zoom” needs just 1/250th second. Nice. And the fast prime lens needs only 1/800th second: a stunningly fast shutter speed that will freeze any motion.

The above images, and those numbers, show very clearly why I would rather shoot a party with a prime f/1.4 or f/2.0 lens than with a zoom lens. If there is one investment you might want to consider making it is that fast lens.

Here’s an effect we forget sometimes. When a lens is wide open, it vignettes.

My 50mm lens at f/1.2:

And here is that same lens a stop and a third closed down, at f/2.0:

Can you see the difference? The first picture, wide open, shows significantly more vignetting.

Now I like vignetting – a lot, in portraits. But shooting portraits with a lens wide open is rather dangerous, since depth of field is very shallow and may not be sufficient. So I add vignetting in Lightroom – Post Crop Vignetting is one of the best controls in Lightroom for when you are shooting portraits.

And when you are not shooting portraits, avoid vignetting like this – so in those cases, avoid shooting with your fast lens wide open.